1. Field of the Invention
The present invention relates to an image heating apparatus of an image forming apparatus, which uses an electrophotographic process, such as a copying machine, a laser printer, a facsimile and the like.
2. Related Background Art
Recently, a fixing apparatus has adopted a film heating process. The film heating process, as it is disclosed in Japanese Patent Application Laid-Open No. 63-313182, first produces a heater by forming a pattern of a heating body on a ceramic substrate, and then heats a body to be heated by making the heater generate heat through a cylindrically formed thin film placed between the heater and the body to be heated.
If an endless belt-like film is used in such a film heating process, a large approaching force is produced on the film. As a measure to the approaching force, a method disclosed in Japanese Patent Application Laid-Open Nos. 4-44075 and 4-44077 has been put to practical use. In the method, the film is wound loosely and driven for the reduction of the approaching force of the film and for the decrease of the driving torque therefor.
FIG. 7 shows the configuration of the pressurizing portion of An image heating apparatus being related art of the present invention. In FIG. 7, a reference numeral 101 designates a heat-proof film; a reference numeral 103 designates a heater holding member; and a reference numeral 104 designates a metal stay. A reference numeral 127 designates a compression spring, and a reference numeral 105 designates a pressurizing force transmitting member, which is made of a resin member and is provided with a pressurizing portion 105a. The lower end portion of the compression spring 127 touches the top surface of the pressurizing portion 105a. A reference numeral 106 designates a pressure roller, and a reference numeral 128 designates a spring pressing member, which the upper end portion of the compression spring 127 touches. A reference numeral 131 designates a pressure roller supporting member having a notch portion. The upper portion of the notch portion is formed to be an opening. A pressure roller bearing 132 for supporting the pressure roller 106 by the shaft thereof is fitted to the lower portion of the notch portion.
As shown in FIG. 7, the heat fixing apparatus is configured to make it possible to press a heater 102 (see FIG. 8) against the pressure roller 106 by pressurizing the pressuring portion 105a of the pressuring force transmitting member 105 to the pressure roller 106 with the compression spring 127, with the metal stay 104 and the heater holding member 103 being put between the pressuring portion 105a and the heater 102.
The spring pressing member 128 has the function of pressing the upper end of the compression spring 127. However, if the spring pressing member 128 only satisfied the function, the spring pressing member 128 might integrally be formed with the pressure roller supporting member 131.
However, if the pressure roller supporting member 131 and the spring pressing member 128 were integrally formed, the pressurizing force transmitting member 105 could not inserted into the pressure roller supporting member 131 from the direction of an arrow A shown in FIG. 7 when the pressurizing force transmitting member 105 would be incorporated into the pressure roller supporting member 131. Besides, when the compression spring 127 would be assembled, the compression spring 127 should be inserted from aside into the space formed by the integrally configured pressure roller supporting member 131 and the spring pressing member 128 while the compression spring 127 should be being compressed. Accordingly, for avoiding such problems concerning the operability of assembling, the pressure roller supporting member 131 and the spring pressing member 128 are formed as separate parts such that the compressing spring 127 and the spring pressing member 128 can be assembled from the direction of the arrow A shown in FIG. 7 after the pressurizing force transmitting member 105 has been incorporated into the pressure roller supporting member 131.
Moreover, it is general that a projection (a paring or a drawing having a height of a degree of the thickness of a sheet metal) is formed on each of the spring pressing member 128 and the pressurizing force transmitting member 105 for locating the compression spring 127.
However, the heat fixing apparatus shown in FIG. 7 has the following problems.
A first problem is the inferior efficiency of the assembling of the spring pressing member 128. The pressurizing force of the compression spring 127 is generally 10 kg or more. And it seems to be inevitable to improve the thermal efficiency of the heat fixing apparatus in consideration of the recent demand of the market of increasing the printing speed of a laser printer and of improving the fixing property thereof. Consequently, the pressurizing force has the tendency of being increasing furthermore afterwards.
Moreover, the spring pressing member 128 is generally incorporated into the pressure roller supporting member 131 as follows. That is, one method is, as shown in FIG. 8A, to fit one end side of the spring pressing member 128 to the pressure roller supporting member 131, and then to rotate the other end side of the spring pressing member 128 against the spring force of the compression spring 127 around the fitted end side thereof while incorporating the spring pressing member 128 to the pressure roller supporting member 131. Another method is, as shown in FIG. 8B, to slide the spring pressing member 128 against the spring force of the compression spring 127 while incorporating the spring pressing member 128 in the pressure roller supporting member 131. The largeness of the spring load exceeding 5 kg on one side is a factor making the operability at the assembling of the spring pressing member 128 worse.
A second problem of the heat fixing apparatus shown in FIG. 7 concerns the spring constant of the compression spring 127. That is, the spring constant cannot be made to be large owing to the assembling method thereof described above.
It is needless to say that the spring constant of the compression spring 127 is desirable to be as small as possible for attaining the stable pressurizing force of the compression spring 127 to make the fixing property of the heat fixing apparatus stable from consideration of the hardness of the pressure roller 106, the load of the compression spring 127, and the dispersion of the accuracy of the dimensions of each part at the mass production thereof.
However, if the spring constant is made too small, the free length of the spring before the assembling thereof becomes long as a result. Thereby, the compression spring 127 is buckled in the assembling process thereof. Consequently, the problem concerning the inferiority of the assembling efficiency is further promoted.
Moreover, the loss of the spring force of the compression spring 127 owing to the buckling thereof frequently becomes a problem when the free length thereof becomes long as a result of the reduction of the spring constant. FIG. 9A shows the situation. Generally, it is said that the loss of the spring force is easily produced when the rate of the free length of a compression spring to the winding diameter thereof exceeds four. Consequently, the free length is normally set within the range of the condition of the rate, and the minimum spring constant is generally determined on the basis of the set free length.
A method for decreasing the loss of the spring force owing to the buckling, as shown in FIG. 9B, provides a guide bar 128a in the inner diameter portion of the compression spring 127 to prevent the buckling thereof. However, in the case where the compression spring 127 and the spring pressing member 128 are rotated or slid while being incorporated in the pressure roller supporting member 131 as shown in FIG. 8A and FIG. 8B, it is impossible to provide the guide bar 128a. Consequently, in the heat fixing apparatus, the pressurizing portion of which is assembled by a method shown in FIG. 8A or FIG. 8B, the spring constant of the compression spring 127 could be made small within the range in which the loss of the spring force thereof owing to the buckling thereof was not produced.
A third problem of the heat fixing apparatus shown in FIG. 7 concerns the spring pressing member 128 being a metal part. Because the spring pressing member 128 supports a load exceeding 5 kg as described above, it is necessary that the bending rigidity of the spring pressing member 128 in the direction of an arrow B in FIG. 7 should be large. Consequently, it is essential that the material of the spring pressing member 128 in the shape shown in FIG. 7 should not be a resin but a metal. Hence, the metal spring pressing member 128 has the following problems. That is, the sizes of the parts thereof become large; the manufacturing cost thereof becomes high; the fixing means thereof is also need high rigidity; and such a guide bar as is shown in FIG. 9B is difficult to form.
An object of the present invention is to provide an image heating apparatus capable of improving the operability thereof at the assembling thereof.
Another object of the present invention is to provide an image heating apparatus including: a heater; a film sliding on the heater; a roller forming a nip with the heater with the film being put between the roller and the heater; an elastic member for pressing the heater to a side of the roller; a pressing member for pressing an end portion of the elastic member on a side thereof opposed to a side of the heater; and a supporting member for supporting the pressing member, wherein a recording material bearing an image thereon is nipped and conveyed at the nip with the image on the recording material being heated by heat from the heater through the film, and the supporting member has an opening portion through which the elastic member can pass.
A further object of the present invention will be clear in the following description.